Defective cellular immune function in individuals with the Acquired Immunodeficiency Syndrome (AIDS) is associated with their development of opportunistic infections and tumors. The agent(s) responsible for this syndrome and the mechanism accounting for the immune incompetence in these individuals remain unknown. We have characterized a naturally occurring immunosuppressive disease in Macaca cyclopis monkeys. We will use these animals to elucidate the nature of the cellular defects in AIDS. We will furthermore utilize these monkeys in studies to isolate causative agents in AIDS. Finally, we will utilize a mouse model to analyze the mechanism accounting for decreased immune function in animals after multiple viral infections.